Fracturing is the process of creating fractures in an oil or gas formation to increase productivity of the oil or gas well. Increasing flow surface area of the producing formation allows for better flow of the hydrocarbons. Hydraulic fracturing involves injecting high-pressure fluids into a formation under sufficient pressure to fracture the rock formation that holds the hydrocarbons. A proppant, like glass beads or other particles, is added to the fluid used to maintain the fracture in the formation so that when the pressure is decreased, the fractures will not close completely. This allows for increased flow within a given formation. Hydraulic fracturing can be used on most formations, and it is sometimes combined with acidizing.
However, reservoir engineers have determined that many wells still produce at less that their potential production rates, even after having undergone the fracturing process. This is because the formation in the vicinity of the wellbore has suffered some form of damage which effectively results in a barrier or "skin" being formed at the interface of the wellbore with the producing formation. Darley & Gray (1988) Composition and Properties of Drilling and Completion Fluids, 5th Ed., Gulf Publishing Co., Houston, page 481.
"Skin effect" is a measurement estimating the restriction to flow through the reservoir walls adjacent the wellbore. Positive values of skin effect indicate damage to the formation adjacent the wellbore resulting in reduced permeability.
Although the contaminated or damaged zone causing the skin effect may only extend a few feet into the formation, it can cause quite large reductions in productivity by reducing the reservoir pressure at the radius of interface with the wellbore. Since flow system is pressure driven, an artificially lower pressure at the wellbore formation interface results in a less than optimal production rate from the well. This is because fluid flow in this system is essentially radial, and therefore, the pressure lost across the damaged zone is proportional to the natural log of the damage zone (rd) divided by the radius of the well (rw). Because the pressure differential across the damage zone is logarithmic, even small changes in the ratio of the radius of the damage (rd) to the radius of the well (rw) can have a significant influence on the pressure drop across the damage zone. Therefore, any technology which can reduce the skin effect by decreasing the amount of formation damage which defines the radius of the damage zone (thereby increasing permeability) is beneficial to the industry by increasing production rates and helping prevent the waste of otherwise unrecoverable petroleum resources. There are many causes of formation damage and the resultant skin effect including: fluid invasion, solids invasion, emulsions, clays hydration, change of metability and in situ precipitation.
One source of production damage resulting in positive skin effects occurs when there is an inherent physical or chemical incompatibility between the fluid used prior to and subsequent to the fracturing fluid to finish the well. Completion fluids are used to displace drilling fluids and other debris, which otherwise could block the pores of the producing strata causing skin effect. Examples of completion fluids are disclosed in Block, U.S. Pat. No. 4,541,485; Stauffer et al., U.S. Pat. No. 4,490,262; and Grimsley, U.S. Pat. No. 4,959,165.
Typically, during the completion process, some completion brine is lost to the formation prior to fracturing the well. Also, after the fracturing process, residual fracturing fluids remain in the wellbore, pores and channels of the formation strata. If the completion fluid and the fracturing fluid are not fully compatible, their incompatibility may manifest when the two fluids interact. The interaction occurs from physical contact of frac fluids with brine previously lost into the formation and completion fluid that follows the frac fluids. Incompatibility between the fracturing fluid and the completion fluid can result in the formation of precipitants, emulsification, and changes in viscosity.
Insoluble precipitants can cause damage similar to that caused by solids invasion, where the precipitants become trapped and compacted in the pores and channels of the formation. Emulsification can occur as well as the formation of offgases creating gas bubbles, which become trapped in the formation, and cause capillary resistance to flow or block pore openings. Changes in viscosity, particularly increases in viscosity, can reduce the efficiency of the completion fluid, and cause microenvironments of high pressure, which can damage the formation.
Any of these manifestations of incompatibility, alone or in combination, can be a source of skin effect. Bridges, U.S. Pat. No. 4,938,288 at col. 1 lines 28-34, teaches that compatibility of the fluids present in the Formation with the completion fluid is one of four factors determining the possibility and degree of skin effect. Bridges discloses a method of adding an acidic buffer composition to a calcium-based completion fluid.